A combination or group of words that gives a complete meaning, thought or idea is called a sentence.

Example. 
I ate an apple.

The above combination of words gives a complete meaning or idea. Such a group words can be called a sentence.

Now, see another combination of words “table he a bought” or “mango read car big”. These combinations of words do not express a complete meaningful sense to a reader or a listener. Hence, such a combination of words cannot be termed as a sentence.

A sentence must have all the words in proper sequence to reveal a proper complete meaning. A sentence can have nouns, adjectives, verbs, subject, object and article in correct sequence according the rules of English grammar. A sentence starts with a capital letter (first alphabet of first word) and ends with full-stop (or period), exclamation mark or question mark.

Here are examples:

• She is singing a song.
• He is a doctor.
• Kids are playing in the garden.
• He will come here tomorrow.
• We watched a movie.
• What is your name?
• What a pleasant weather!
• It has been raining since 3 O’clock.
• I am learning how to drive a car.
• She wrote him a letter.
• They bought a new computer.
• There ten students in the classroom.
• Where had you gone the last night?
• The birds are flying in the sky.
• He got admission in a new school.